The invention relates generally to a method and apparatus for generating coherent near millimeter wavelength radiation. More particularly, the invention relates to a diffraction radiation generator having a depressed collector, in which elements used to decelerate the electrons in the ribbon electron bean can also be used to analyze the ribbon beam.
In a diffraction radiation generator, such as an orotron, a ribbon electron beam is generated by applying a voltage between the cathode and anode of an electron gun. Generally, the anode is grounded and a negative voltage is applied to the cathode. The ribbon electron beam is directed across a slow wave structure within an open resonator to a collector disposed on the opposite side of the open resonator from the electron gun. In an orotron, the open resonator is formed by two mirrors and the slow wave structure is a reflecting diffraction grating embedded in one of the mirrors. If the proper conditions of synchronism between the electron velocity and the phase velocity of an evanescent wave traveling along the diffraction grating are met, coherent radiation results. The electron velocity is controlled by the potential difference between the cathode and the diffraction grating; the latter is generally near anode or ground potential.
In the past, the collectors of diffraction radiations generators have been maintained at or very close to the anode potential. Thus, the velocity of the electrons forming the ribbon beam remains essentially unchanged after these electrons pass through the open resonator, and all of the kinetic energy of these electrons is converted to heat when these electrons strike the collector. It would be highly desirable if these electrons could be decelerated before striking the collector, thus recovering most of their kinetic energy and improving the efficiency of the diffraction radiation generator.